[v1] Required fields in an optional object are required even if the optional object isn't defined

Describe the bug This bug is similar to #1890 where optional objects with required fields are causing unexpected behavior when the optional object isn't provided. However, this issue documents a problem with the latest version of yup and a different error.

To Reproduce

It seems the CodeSandbox tests panel was removed, but here is a test that should pass, but the objWithoutFoo will fail.

const yup = require("yup");

it("should pass", async () => {
  const schema = yup.object({
    fooValid: yup.boolean().required(),
    foo: object({
      str: yup.string().required(),
      num: yup.number().required(),
      bool: yup.boolean().required(),
    }).optional(),
  });

  const objWithFoo = {
    fooValid: true,
    foo: {
      str: "hello world",
      num: 42,
      bool: true,
    },
  };

  await expect(schema.isValid(objWithFoo)).resolves.toBe(true);

  const objWithoutFoo = {
    fooValid: false,
  };

  await expect(schema.isValid(objWithoutFoo)).resolves.toBe(true);
});

Expected behavior I expect all of the provided tests to pass. Required fields in an optional object should only be required if the optional object is defined.

Platform (please complete the following information):

NodeJS
Version 1.4.0

Additional context N/A

jquense / yup

[v1] Required fields in an optional object are required even if the optional object isn't defined #2215